Method of making metal castings

ABSTRACT

Method of and apparatus for making metal castings using a polystyrene pattern 10 embedded in unbonded sand 20 which is consolidated to form a mould M in which is defined a mould cavity C. Molten metal is fed into the cavity C and permitted to solidify to form a casting 11. The metal is aluminum or an aluminum alloy and at least part of the mould is formed of sand which comprises at least 50% zircon sand, or other particulate material suitable for making a mould and having a bulk density lying in the range 2-3 grams/cc. The mould M may have a moulding feature 9 having upper and lower surfaces for contact with the metal and which projects inwardly of the mould cavity C from the main wall thereof and the length of any one section of the molding feature 9 being at least twice the thickness of the thinnest part thereof by which the section is connected to the main wall of the mould cavity.

This invention relates to a method of making metal castings using thefull mould process, namely providing an in situ destroyable pattern of,for example, expanded polystyrene, embedding the pattern in unbondedsand, consolidating the sand to form a mould in which is defined a mouldcavity, feeding molten metal into the cavity, permitting the metal tosolidify within the cavity to form a casting, interrupting the feed ofmetal to the cavity and removing the casting from the cavity. Thepattern may be destroyed in situ either by the metal as it is fed intothe cavity, or by the application of heat prior to introduction of themetal into the cavity.

Problems of lack of accuracy and poor surface finish have beenencountered when using the full mould process to produce high qualitycastings of an aluminium or an aluminium alloy.

An object of the present invention is to overcome this problem.

In accordance with the present invention I provide a method of makingmetal castings comprising the steps of providing an in situ destroyablepattern, then embedding the pattern in unbonded sand and consolidatingthe sand to form a mould in which is defined a mould cavity, thenfeeding molten metal into the cavity and permitting the metal tosolidify within the cavity to form a casting and interrupting the feedof metal to the cavity and removing the casting from the cavity, whereinthe metal is aluminum or an aluminum alloy and at least part of themould being formed of said which comprises at least 50% sand as hereindefined, by weight of sand.

Said part may comprise at least one moulding feature, made of unbondedsand, as hereindefined.

Said part may comprise unbonded sand and comprise the whole orsubstantially the whole of the mould.

Said mould cavity may comprise at least one preformed moulding featurewhich is included in said pattern.

Said part may comprise at least one preformed moulding feature which isincluded in said pattern.

Said preformed moulding feature may be as hereindefined.

The whole of the mould, except the or each preformed moulding featuremay comprise unbonded sand which comprises at least 50% sand, ashereindefined, by weight.

The invention also provides, according to another aspect, a method ofmaking metal castings comprising the steps of providing an in situdestroyable pattern, then embedding the pattern in unbonded sand andconsolidating the sand to form a mould in which is defined a mouldcavity, then feeding molten metal into the cavity and permitting themetal to solidify within the cavity to form a casting and interruptingthe feed of metal to the cavity and removing the casting from thecavity, wherein the mould cavity comprises a moulding feature ashereindefined, made of at least 50% sand, as hereindefined, by weight ofsand.

The moulding feature may be formed of unbonded sand, or the mouldingfeature may be preformed of sand bonded with a bonding agent.

In either aspect of the invention, the unbonded sand may comprise whollyor substantially wholly zircon sand, except for usual impurities, whilstthe bonded sand may comprise wholly or substantially wholly zircon sandand bonding agent except for usual impurities.

By virtue of using at least 50% zircon sand and preferably wholly, orsubstantially wholly, zircon sand, I have found an unexpectedimprovement in the quality of the casting produced compared withcastings produced using the full mould process to cast such metals,particularly where a moulding feature as hereinbefore is provided in themould cavity, when the mould is made of other sand, such as silica sand.

Analysis of the system in the light of this unexpected result leads tothe belief that the improvement is due to the density of the sand beingsubstantially the same as the density of the molten metal being cast. Itis believed that this minimises the hydrostatic forces acting on themoulding feature thereby greatly improving mould stability duringcasting and hence greatly improving the final accuracy of the casting.

Because the mould when comprising a moulding feature, as hereindefined,has portions which are relatively thin and extend in cantilever or inbeam a significant distance from the remainder of the mould cavity, thenthese portions are subjected to minimised hydrostatic forces, therebygreatly improving the mould stability. The breaking off of suchportions, which has sometimes been observed when the mould has been madeof other sand, is avoided.

Analysis of the system leads me to believe that the quality of thecasting is further improved because zircon sand produces a morecompacted or consolidated mould, for given compaction techniques, thanother sands. Indeed, the improvement in compaction is such that lesssand head is required for an equivalent mould strength to resist thehydraulic filling loads of the molten metal than is necessary with othersands. This is due to the density of zircon being approximately twicethat of conventional sands.

The method may be performed by embedding the pattern directly in thesand without application of a refractory coating to the pattern.

I have found that when using zircon said it is not necessary to apply arefractory wash to the pattern. This is a significant advantage becausethe usually applied refractory wash, which it is necessary to apply whenusing other sand such as silica sand, is often very difficult to apply,especially when used in cored areas, with which the present invention isspecifically concerned, and is also difficult to remove from theresulting casting.

Moreover, the presence of the conventional refractor coating inhibitseasy removal of the products resulting from the destrution, e.g. bycombustion or vapourisation, of the pattern causing difficulties whenattempting to fill a mould quickly or when filling thin sections of themould.

Alternatively, a non-refractory coating may be applied to the patternprior to embedding in the sand. For example varnish, paint, starch, toimprove the surface finish of the resulting casting. Alternatively thenon-refractory coating may be an exothermic coating applied to aidfilling of thin sections of the mould.

I have found that zircon sand has better mould filling capability andcompaction thereof is easier than other sands. It is believed that thisis because zircon sand has an essentially spherical grain form whichflows more easily than other sand grains.

Zircon sand can be readily cleaned and reclaimed, if necessary, byeconomical thermal reclamation due to its refractory properties withminimal loss due to degradation and greatly reduced dust problems.

The final accuracy of the casting is also though to be contributed to byvirtue of the low and predictable coefficient of thermal expansion ofzircon sand. This is believed to reduce relative movement between themoulding feature and the main wall of the mould to negligible amounts,so giving rise to the exceptional accuracy and reproducability which hasbeen achieved by the present invention. It also greatly reduces scuffingbetween the moulding feature and the cast metal during solidification,thereby giving a greatly improved surface finish compared with thatattained with other sands.

It has also been noted that the as cast mechanical properties of thecasting produced according to the invention are improved compared withthose attained with other sands and this is due to the heat capacity ofzircon sand which gives faster casting cooling rates, indeed rates whichare similar to those attained with conventional metal dies.

Provision of a preformed moulding feature retains all the advantages ofconventional casting using an in situ destroyable pattern such asabsence of flash, economy and the like but provides the followingadditional advantages. Hitherto, when using such a pattern for castingswith relatively complicated internal cavities, the pattern has been madeby securing together, by means of an adhesive, a plurality of relativelythin slices of pattern each of which is formed with the desired portionof cavity. Typically the complete pattern has comprised five slices. Ithas been necessary to age and condition these slices individually andthen accurately to assemble and glue them together without using excessglue or too little. It has proved to be a considerable problem inpractice to achieve this because the individual slices can change shapeon aging or conditioning and it can be difficult or impossible toassemble them sufficiently accurately. In addition, unless the preciselycorrect amount of adhesive is applied, if excess is applied then it isexuded from the joint lines and is either necessary to remove the glueor the glue forms defects in the casting and it is generally notpossible to remove any such excess glue from the internal cavities. If,on the other hand, to avoid this the adhesive applied is minimised,there is a danger that insufficient adhesive would be applied thuspermitting separation of the slices.

Moreover, extensive problems have been encountered in achieving fillingof the cavities with the unbonded sand when the pattern is embeddedtherein.

The preformed moulding feature or features, such as cores, areaccurately dimensioned and since the accuracy of the casting isdetermined by the accuracy of the internal cavity, then the accuracy ofthe casting is dimensionally controlled by the moulding feature orfeatures, in this example cores. In addition the problems arising fromexcess glue are avoided both internally and externally and of coursethere are no problems in achieving filling of the cavities with sand.

Moreover, the in situ destroyable pattern can be used almost immediatelyafter blowing since it does not need to be aged or conditioned becausethe accuracy of the casting is controlled by the preformed mouldingfeature or features.

Other features of the invention will now be described.

The pattern may be destroyed in situ by the heat of the metal as it isfed into the mould cavity.

Alternatively the pattern may be destroyed in situ prior to feeding themetal into the mould cavity.

The pattern may comprise a casting part to provide a casting portion ofthe cavity and an ingate part to provide a casting ingate portion of thecavity.

The pattern may also be provided with a runner system part to provide arunner system portion of the cavity and a runner ingate part to providea runner ingate portion of the cavity.

The metal may be fed from source into the mould through an orifice in acontainer for the mould.

The casting ingate part of the pattern may be disposed in castingrelationship with the orifice and then the sand may be introduced aroundthe casting part and casting ingate part to embed the pattern within thesand.

The runner ingate part of the pattern may be disposed in castingrelationship with the orifice and then the sand may be introduced aroundthe casting part and runner system and runner ingate parts to embed thepattern within the sand.

The pattern may be supported within a container by means of a feedmember which is mounted within the container, and sand may be introducedinto the container to embed the pattern therein. Thereafter, metal maybe fed through a passage provided by the feed member into the mouldcavity.

The pattern may be disposed within a peripheral wall extending upwardlyfrom a mould base to provide a mould box or container into which thesand is introduced to embed the pattern therein.

The pattern may be coated with a refractory coating which may improvesurface finish.

Preferably, the pattern is uncoated or may be coated with anon-refractory coating to improve surface finish, for example varnish,paint or starch.

The sand in which the pattern is embedded may be consolidated byvibration or the application of a vacuum, or by other means, or by acombination of such means.

A pressure below atmospheric pressure may be applied to the mould duringcasting to assit consolidation and/or removal of vapour or otherdecomposition products of the pattern.

The metal may be fed downwardly under the force of gravity from a sourceof molten metal into the mould cavity.

Alternatively, and preferably, the metal may be fed generally upwardlyagainst the force of gravity from a source of molten metal into themould cavity.

The source of molten metal preferably comprises a reservoir of moltenmetal which is at a level which is below the level of the cavity.

The metal may be fed into the mould cavity through a passage having anend surrounded by the molten metal in the source, an opposite end whichis connected to the mould cavity and an intermediate part which extendsthrough the free upper surface of the molten metal in the source.

A pump may be provided to pump metal upwardly from the reservoir intothe cavity through the passage.

The metal may be pumped into the cavity at the bottom thereof.

The metal to be cast may be supplied to the reservoir by feeding metalin solid state therein to, and melting the metal in the reservoir.

The reservoir may have a feed region whereat said metal is fed into thereservoir in solid state, and a casting region from which metal, inliquid state, is drawn by said pump.

The reservoir may have a heating region, between the feed region and thecasting region in which heat is applied to the metal in the reservoir.

Alternatively the metal to be cast may be applied to the reservoir inmolten state from a source of molten metal separate from the reservoir.

The metal may be supplied to the reservoir by means of a ladle.

The metal may be supplied to the reservoir by means of a launder.

The metal may be supplied to the reservoir from a melting furnaceseparate from the reservoir.

The metal may be pumped by an electro-magnetic pump or by a fluidpressure pump.

Alternatively the metal may be pumped by providing the reservoir withina sealed housing and pressurising the interior of the housing to forcemetal upwardly through a riser tube extending from below the level ofmetal in the reservoir through the housing.

After the metal has solidified, the level of metal in the riser tube maybe lowered below the level of the entry to the mould and thereafter themould and casting are removed from casting relationship with the sourceof metal, together with the mould base.

The casting may be removed from the mould by tipping out the sand or byfluidising the sand or by any other desired means.

After removal of the casting from the mould, the ingate and any otherrunning system and feeding system, if present, may be removed from thecasting.

The mould cavity may be filled by a flow of metal generally upwardlyagainst the force of gravity throughout the mould cavity.

The mould cavity may be filled without any substantial flow of the metaldownwardly under the influence of gravity within the mould cavity.

The metal may be fed into the mould cavity by a low pressure deliverysystem, which causes a differential pressure to exist between thepressure in the mould cavity and the pressure in the source of moltenmetal.

Said differential pressure may be in the range of 0.1 to 1.0 atmospheresand preferably 0.20 to 0.70 atmospheres.

The mould cavity may comprise at least one casting portion, in which afinal casting is produced, and metal is fed to the casting portion at asingle location and the casting portion is designed so that no partthereof is fed from another part of the casting portion along a pathhaving any substantial flow downwardly under the influence of gravity.

The mould cavity may comprise at least one casting portion, in which afinal casting is produced, and metal is fed into the casting portion ata plurality of locations so that the casting portion is filled bygenerally upward flow of metal from a plurality of locations against theforce of gravity without any substantial flow of metal downwardly underthe influence of gravity.

The mould cavity may include a casting ingate portion which communicatesdirectly with the casting portion.

The casting ingate portion of the cavity may communicate with a runnersystem portion of the cavity which is provided with a runner ingateportion of the cavity which communicates with the source of metal.

The casting ingate portion may communicate with a source of metalwithout any runner system.

The ingate may be placed in casting relationship with the orifice byinserting a portion of the ingate part of the pattern into close fittingengagement within the orifice.

The orifice may be lined with, or integrally formed in, thermallyinsulated refractory material capable of withstanding the liquid metalto be cast.

The orifice may be reused for a plurality of castings.

Alternatively the orifice may be disposed of after each castingoperation.

The orifice may be formed as an insert in the mould base.

The orifice may be placed in casting relationship with the source ofmetal and a feed is effected by the use of a ceramic fibre gasketbetween a riser tube extending between the source of metal and themember in which the orifice is formed.

Said feeding of molten metal generally upwardly against the force ofgravity from the source of molten metal into the mould cavity may beperformed without any substantial flow of metal downwardly under theinfluence of gravity between the source and the entry into the cavity.

The filling defects which are encountered when metal is allowed to fallunder gravity to fill a mould cavity arise because of the action of theliquid metal whilst it falls downwards under gravity. The uncontrolledtumbling, splashing, surging etc., introduces and entraps oxides, gasesand decomposition products from the pattern and mould materials into themetal. Even when the flow is more gentle, cool streams of metal developa carbon deposit from decomposing styrene vapour, which prevents towsuch streams from effectively merging in parts of the casting.

By feeding metal upwardly against the influence of gravity I have foundthat the above mentioned problem is overcome or reduced because thegentle rise of the substantially horizontal metal surface keeps themetal separate from and unmixed with the decomposable pattern and itsdecomposition products since decomposition of the pattern occursprogressively ahead of the advancing metal surface.

By an "in situ destroyable pattern" I mean a pattern which, when in asolid state, is sufficiently strong to enable the sand to be formedtherearound and which can be destroyed in situ so as to leave a mouldcavity. For example, the pattern may be destroyed in situ by being atleast substantially completely transformed to the gaseous state, whilstwithin the sand and the sand permitting the transformation products toleave the cavity. The pattern may be subjected to heat to cause it tovaporise and/or burn and/or undergo some other chemical reaction. Oneexample of a suitable destroyable pattern is a pattern made of expandedpolystryene which is decomposed by combustion substantially to thegaseous state on heating. Such a pattern is commonly known as anevaporative pattern. Of course some of the decompositon products may besmall solid particles such as soot but these can leave the mould cavitytogether with the gaseous products of combustion, for example by passingthrough the pores between the particles of the particulate material.Although it is preferred that the pattern is destroyed in situ byutilising the heat of the molten metal as it is cast into the mould, ifdesired, the pattern may be pre-destroyed in situ, for example, byapplying heat to the pattern prior to casting.

By a "moulding feature as hereindefined" I mean a moulding feature whichhas upper and lower surfaces for contact with the metal and whichprojects inwardly of the mould cavity from a main wall thereof and has aconfiguration such that, if the mould were made of 100% silica sand,except for usual impurities, and LM25 aliminum alloy were the metalcast, a part of the casting resulting from the moulding feature isdisplaced by at least 5% from its designed position relative to thenearest part of the casting defined by the main wall of the cavity. Forexample, the length of any one section of a moulding feature may be atleast twice the thickness of the thinnest part thereof by which thesection is connected to the main wall of the mould cavity.

By "sand as hereindefined" I mean zircon sand, or other particulatematerial suitable for making a mould and having a bulk density lying inthe range 2-3 gm/cc.

According to another aspect I provide a mould for making metal castingscomprising consolidated unbonded sand in which is embedded an in situdestroyable pattern to define a mould cavity, there being provided inthe mould cavity a moulding feature as hereindefined and comprising atleast 50% sand as hereindefined, by weight of sand and, optionally, anyother feature of the mould disclosed or claimed herein.

Three embodiments of the invention will now be described by way ofexample, with reference to the accompanying drawings, wherein:

FIG. 1 is a diagrammatic cross-sectional view through part of anapparatus for performing the method embodying the present invention;

FIG. 2 is a perspective view of the pattern for the casting and ingateshown in FIG. 1;

FIG. 3 is a diagrammatic cross-section to a reduced scale through a lowpressure casting machine for use with the apparatus shown in FIG. 1;

FIG. 4 is a diagrammatic cross-section, to a reduced scale, through amelter/holder furnace for use with the apparatus and pattern of FIGS. 1and 2 in a second embodiment of the invention; and

FIG. 5 is a diagrammatic cross-sectional view through an apparatus forperforming a third embodiment of the invention.

FIG. 6 is a diagrammatic cross-sectional view through a pattern forperforming a fourth embodiment of the invention.

Referring to the drawings, a pattern made of expanded polystyrene isindicated at 10 and comprises two parts namely a casting part 11 of adesired shape of the final casting to be produced, and a casting ingatepart 12. The pattern 10 is made in conventional manner by introducingpolystyrene granules into a moulding machine where they are injectedinto a die of the desired configuration. Steam is then injected whichcauses the granules to expand and fuse together. The resultant expandedpolystyrene pattern is then water cooled and ejected from the die.

Although in the example illustrated the pattern is a one-piece mouldingwith the casting parts 11 and 12 integral with each other, dependingupon the shape of the final casting and ingate or ingate and runnersystem, the pattern may be moulded in two or more separate parts bondedtogether by a suitable adhesive or other means.

The pattern is then stored so that the normal pattern shrinkage occursprior to use of the pattern. Of course, the die in which the pattern ismade of correspondingly larger size to allow for the shrinkage both ofthe pattern and of the fina casting.

The pattern 10 is then positioned so that the ingate part 12 is in closefitting engagement with a cylindrical orifice 13 formed in an insert 14made of suitable insulating refractory material such as a lightweightrefractory cement, removably mounted by plates 15 secured in position bybolts 16 in an aperture 17 of a mould base board 18. An open bottomedand topped wall member is then positioned on top of the mould base 18 sothat the pattern 10 is supported within a container 19 by means of theingate part 12, which is mounted within the container by said engagementwith the orifice 13. Then sand 20 is poured into the container 19 aroundthe pattern 10 so as to embed the pattern 10 in the sand 20 and form amould M in which is defined a mould cavity C.

The casting part 11 is shaped so as to provide the mould cavity C with amoulding feature 9 which projects inwardly of the cavity C from a mainwall 8 thereof and has a length L which is in the present example threetimes the minimum thickness T of a part 7 of the feature 9 by which thefeature 9 is connected to the main wall 8.

In the present example, the sand comprises 100% zircon sand and iswithout any binder or any other component except for usual impurities.If desired the sand may comprise up to 50% of sand other than zirconsand such as silica sand and/or olivine sand or any other suitableparticulate material having a bulk density in the range 2-3 gm/cc. Butit is preferred that the sand comprises wholly, or substantially wholly,zircon sand.

The zircon sand has a particulate grain size lying in the range 50μm to500μm. An average grain size of 150μm is common but as low as 75μm isexperienced.

In the present example, the sand is consolidated around the pattern 10by vibrating the assembly of mould base 18, container 19 etc., but itmay be consolidated by any other suitable means such as the applicationof suction to the interior of the mould material, or by other means orby a combination thereof and may be consolidated whilst the sand ispoured into the container 19 as well as, or instead of, thereafter.

The mould base 18 carrying the moulding material 20 and pattern 10therein is then positioned in casting relationship with a conventionallow pressure casting machine Ma so that a riser tube 21 of the machineis placed in sealing engagement with the insert 14 with a ceramic fibregasket 22 therebetween to provide a liquid-tight seal.

The low pressure die casting machine ma comprises a furnace 23 havingelectrical heating elements 24 containing a sealed reservoir 25, towhich molten metal is fed from a separate melting furnace by means of,for example, a ladle. The riser tube 21 provides a passage which has alower end immersed in the molten metal in the furnace, an upper end forconnection to the mould feature by sealing engagement with the insert 14and an intermediate portion which extend through the free, upper,surface of the molten metal. If desired the molten metal may be fed byother means such as a launder. After filling with molten metal thereservoir 25 is sealed and the machine Ma is then operated bypressurising the reservoir 25 in conventional manner by applying gas,e.g. air or nitrogen, under pressure, e.g. 0.2 to 0.7 atmospheres, so asto force metal up the riser tube 21 to cast molten metal into the mouldcavity C through a feed member provided by the casting ingate part 12.

If desired, metal may be fed from a holding furnace which need not besealed from the atmosphere by using a pump separate from the reservoirsuch as an electro-magnetic pump of a fluid pressure pump.

In the present example, the metal is an LM25 aluminum alloy, but may beany other aluminum alloy or pure aluminum. The molten metal is fed bythe casting machine through the riser tube 21 and into the orifice inthe insert 14 where the heat of the metal causes progressivedecomposition of the ingate part 12 and casting part 11 so that thepattern 10 is destroyed by being decomposed into gas and/or small solidor liquid particles which escape from the resultant cavity through thepores between the particles of sand 20. Thus, the molten metal occupiesthe mould cavity C in the sand 20 which was previously occupied by thepattern 10.

If desired, a partial vacuum may be applied to the mould during at leastthe initial stages of feeding metal into the mould to assist withconsolidation and/or removal of vapour or other decomposition productsof the pattern.

After the mould cavity C has been filled with liquid metal, the metal isallowed to solidify, or at least solidify to the extent so as to beself-supporting. Pressure is then released or partially released toallow the metal to fall back or partially fall back from the level ofthe ingage down the riser tube into the reservoir, and then the mouldand the casting therein are removed out of casting relationship with thecasting machine Ma together with the mould base 18 and thereafter thecasting is removed from the moulding material, either by tipping thesand out of contact with the casting or by fluidising the sand to permitit to flow or by other means.

The ingate is then removed from the casting.

Although in the present example, the orifice 13 is formed in a removableinsert 14, if desired, the orifice may be formed in other material thaninsulating refractory material but be lined with insulating refractorymaterial. For example the orifice may be defined in a sleeve of theinsulating refractory material provided in an opening in an aluminumplate mounted on, or which itself forms the mould base 18. The insert 14may be used for a considerable number of casts or replaced after eachcase or a small number of casts depending upon the metal being cast andthe material of which the orifice is made.

In the present example, the casting ingate is placed directly in castingrelationship with the riser tube. If desired, however, in any particularcasting where feeding is required to a plurality of locations to ensurethat the casting is fed by movement of metal upwardly against theinfluence of gravity, a plurality of casting ingates may be providedinterconnected to a runner system along with which the molten metalpasses against the force of gravity without any substantial flowdownwardly under the influence of gravity, and the runner system itselfhaving a runner ingate which is placed directly in casting relationshipwith the riser tube and serves as a feed member to support the patternwithin the container.

Alternatively, a plurality of separate castings may be made at the sametime by feeding molten metal thereto by a similar feeder systemextending from the feeder ingate to a casting ingate of the cavity foreach casting. Alternatively, more than one riser tube may be provided tofeed the metal to feeder ingates corresponding to the number of risertubes. Each feeder ingage may comprise also a casting ingate or eachfeeder ingate may be connected to a plurality of casting ingates by arunner system.

In the second embodiment of the invention the metal, method, pattern andapparatus are as described in connection with the first embodiment,except that, instead of feeding molten metal into the moulds using themachine shown in FIG. 3, there is used the apparatus shown in FIG. 4 anda different shape of pattern is shown.

In this embodiment, referring particularly to FIG. 4, there is provideda melter/holder furnace 30 comprising a refractory lined vessel 31having a generally rectangular base 32, and vertical side and end walls33, 34 respectively. A roof 35 extends across the whole width of thevessel 31 but stops short of the end walls 34 to provide a charging well36 and a pump well 37 at opposite ends of the vessel.

The roof 35 comprises a generally horizontal rectangular top part 38 andvertical side and end walls 39, 40 respectively. The roof 38 comprisessuitable refractory material and within the roof are provided electricalradiant beaters 41.

The temperature of the heaters 41 and the number thereof and the area ofthe top part 38 of the roof are arranged so as to provide sufficientheat to melt ingots fed into the vessel 31 at the charging well 36 andto maintain the metal molten in the remainder of the vessel. Adownwardly depending refractory wall 42 is provided at the charging wellend of the vessel 31 to separate the charging well from the main heatingpart of the vessel whilst downwardly depending and upwardly extendingrefractory walls 43, 44 are provided at the pump well end of the vesselto define a casting vessel region 45 within which a pump 46 is provided.In the present example the pump 46 is an electro-magnetic pump whichpumps metal from the region 45 through a riser tube 47 which isconnected to the mould base 18 in exactly the same way as the riser tube21 shown in FIG. 1. If desired a filter 48 may be provided between thewalls 43 and 44 to filter metal entering the casting vessel 45.

The riser tube 47 and pump 46 provide a passage which has a lower endimmersed in the molten metal in the furnace, an upper end for connectionto the mould by sealing engagement with the insert 14, and anintermediate portion which extends through the free upper surface of themolten metal. If desired, other types of pump, separate from thefurnace, may be used, such as a fluid pressure pump or the furnace mayitself be pressurized analogously to the first embodiment to feed metalinto the mould cavity.

In this embodiment the pattern has the configuration shown in FIG. 4 andthe ration L:T is 5:1. In other respects the pattern is as described forthe first embodiment and the same reference numerals are used fro thesame parts.

In the examples described above the metal is fed upwardly into the mouldcavity against the force of gravity which is the preferred method forthe reasons explained hereinafter. If desired the mould cavity may bearranged to be filled by feeding metal downwardly under the form ofgravity.

In a third embodiment the metal and pattern are as described inconnection with the first embodiment, except that a different shape ofpattern is shown. In this embodiment the pattern has the configurationshown in FIG. 5. It will be seen that the moulding feature 9 bridgesbetween the opposite sides of the main wall 8 of the cavity andcomprises two sections, the lengths of which are indicated at L1 and L2,each section being connected to the associated side of the main wall 8by a part 7 of the section, the minimum thickness of which is T1, T2respectively. In this example the ratio L1:T1 is 9:1 whilst the ratioL2:T2 is 2.

In this embodiment the pattern 10 is embedded in 100% zircon sand asdescribed hereinbefore in connection with the first embodiment and thesand is consolidated around the pattern again as described hereinbefore.In this case the casting ingate part 12 of the pattern is at the top ofthe pattern and the metal is poured into the mould from a ladle Ladownwardly through the casting ingate part 12. The sand is, of course,held within a container indicated at 19'. If desired the same shape ofmould cavity may be provided in either of the first two describedembodiments and vice versa.

In all the above described embodiments the moulding feature is of such aconfiguration that if the mould were made of 100% silica sand, apartfrom usual impurities, it would be found that a part of each mouldingfeature would be displaced by at least 5% from its designed positionrelative to the nearest part of the main wall of the cavity as shown atD₁, D₂ and D₃, whereas in the present invention such displacement doesnot occur as is demonstrated by the following Examples.

EXAMPLES EXAMPLE 1

The apparatus described with reference to FIGS. 1 to 3 was used to make10 castings of the shape shown in FIGS. 1 and 2. 100% unbonded zirconsand by total weight of material was used as the sand 20 of which themould was made.

The distance D₁ between the surface of the casting corresponding to thesurface S1 of the moulding feature 9 and the surface of the castingcorresponding to the top surface S2 of the pattern was measured for eachcasting and was found to differ by, an average, 2.4% from the desireddistance.

EXAMPLE 2

The same measurements were performed as described in connection withExample 1 but using a mould made of 100% unbonded silica sand by totalweight of material as the material 20 of the mould. In this case theabove mentioned distance D₁ was found to differ, on average, by 15.2%from the desired distance.

EXAMPLE 3

The same measurements were performed as described in connection with theprevious Examples but using 100% unbonded olivine sand by total weightof material as the material 20 of the mould. In this case the dimensionD₁ was found to differ by, on average, 11% from the designed distance.

EXAMPLE 4

The castings in Examples 1-3 were examined for the surface finishachieved. The castings produced in Example 1 reproduced exactly thesurface of the pattern and it was not possible to determine a lesserstandard of finish due to the sand. In Examples 2 and 3 a distinctworsening of the surface finish due to metal penetration of the sand wasobserved on all castings.

It is to be noted that in all the above Examples the pattern used wasunprovided with any refractory coating or wash nor was the patternprovided with any non-refractory wash to improve surface finish. Theabove set out results were attained with a completely uncoated pattern.

EXAMPLE 5

The following mechanical properties of the castings resulting fromExamples 1-3 were determined, the average for each Example being asfollows.

    ______________________________________                                                 0.2% P.S.              Brinwell                                      Example  M.P.A.      Elongation %                                                                             Hardness HB                                   ______________________________________                                        1        270         3          110                                           2        220         1          85                                            3        230         1          90                                            ______________________________________                                    

It will be seen that significantly better mechanical properties wereobtained with Example 1 than with the other Examples.

In all the above examples the metal cast was LM25 aluminum alloy and theexamples were all made from this alloy and with the same heat treatmentof the casting.

The aluminum alloy LM 25 has the following composition:

    ______________________________________                                                     Chemical Composition                                                          Limits (Mass %)                                                  Component    Remainder Al                                                     ______________________________________                                        Si           6.5-7.5                                                          Fe           0.5                                                              Cu           0.1                                                              Mn           0.3                                                              Mg            0.2-0.45                                                        Zn           0.1                                                              Ti           0.2                                                              Ni           0.1                                                              Pb           0.1                                                              Sn            0.05                                                            ______________________________________                                    

Referring now to FIG. 6, in which the same references are used to referto corresponding parts as are used in FIGS. 1 to 5 but with the additionof a " sign.

In this embodiment the moulding feature comprises a core 9" which ispreformed, in conventional manner, in zircon sand which comprises 100%of the sand of the core, except for usual impurities. If desired othersand, as hereinbefore defined, may be used and the zircon or other sandmay comprise down to at least 50% of the sand. Alternatively, but lesspreferably, the core may comprise other sand such as silica or a mixtureof sands. The zircon or other sand is preformed to make the core withthe aid of a bonding agent or binder of conventional type.

The thus preformed core 9" is positioned within a pattern 10" of an insitu disposable material such as expanded polystyrene. This is done, inthe present example, by expanding the polystyrene as described inconnection with the previous embodiments in a die of a moulding machinein which the preformed core is located so that it is positioned in thepattern in the desired location.

The combined pattern 10" and preformed core 9" are then used to form amould cavity C" in a mould M" which is made of 100% zircon sand exceptfor usual impurities, (but which may be of any suitable particulatematerial when made in accordance with the second aspect of theinvention) and the casting made as in the previously describedembodiments. That is to say, the metal may be fed into the mould M"upwardly as described with reference to FIGS. 1 to 3 or FIG. 4, ordownwardly as described with reference to FIG. 5 and details of themethod and apparatus, except for the pattern, are as describedpreviously.If desired more than one core or other moulding feature maybe thus provided.

As the metal is cast the expanded polystyrene is replaced by the moltenmetal and the preformed sand core or cores define the internalconfiguration of the casting and are removed in conventional mannerafter the casting has solidified.

Although FIG. 6 shows the moulding feature as a core 9" bridging acrossthe mould cavity C", the moulding feature may be of any desired shape orshapes and may be as hereindefined or of other shape or shapes fallingoutside the above definition and may be connected to the main wall ofthe cavity C" at only one position.

The extent to which the moulding feature extends into the unbonded sandof the mould may differ from that described hereinbefore and indeed maynot extend into the unbonded sand to any significant extent or at all.

The features disclosed in the foregoing description, or the accompanyingdrawings, expressed in their specific forms or in terms of a means ofperforming the disclosed function, or a method or process for attainingthe disclosed result, may, separately or in any combination of suchfeatures, be utilised for realising the invention in diverse formsthereof.

I claim:
 1. A method of making metal castings comprising the steps ofproviding an in situ destroyable pattern, then embedding the pattern inunbonded sand and consolidating the sand to form a mould in which isdefined a mould cavity comprising at least one preformed mouldingfeature which is included in said pattern, then feeding a molten metalselected from the group consisting of aluminum and aluminum alloys intothe cavity and permitting the metal to solidify within the cavity toform a casting and interrupting the feed of metal to the cavity andremoving the casting from the cavity, wherein at least part of the mouldis formed of sand which comprises at least substantially wholly sandselected from the group consisting of zircon sand and other particulatematerial suitable for making a mould having a bulk density lying in therange 2-3 gm/cc.
 2. A method according to claim 1 wherein said preformedmoulding feature has upper and lower surfaces for contact with the metaland projects inwardly of the mould cavity from a main wall thereof andhas a configuration such that a mould made of 100% silica sand, exceptfor usual impurities, and LM25 aluminum alloy is the metal cast, a partof the casting resulting from the moulding feature is displaced by atleast 5% from its designed position relative to the nearest part of thecasting defined by the main wall of the cavity.
 3. A method according toclaim 1 wherein the whole of the mould, except the or each preformedmoulding feature, comprises unbonded said which comprises at leastsubstantially wholly sand, selected from the group consisting of zirconsand and other particulate material suitable for making a mould having abulk density lying in the range 2-3 gm/cc.
 4. A method according toclaim 1 wherein said part comprises at least one moulding feature whichhas upper and lower surfaces for contact with the metal and whichprojects inwardly of the mould cavity from a main wall thereof and has aconfiguration such that the length of any one section of the mouldingfeature is at least twice the thickness of the thinnest part thereof bywhich the section is connected to the main wall of the mould cavity. 5.A method of making metal castings comprising the steps of providing anin situ destroyable pattern, then embedding the pattern in unbonded sandand consolidating the sand to form a mould in which is defined a mouldcavity, then feeding a molten metal selected from the group consistingof aluminum and aluminum alloys into the cavity and permitting the metalto solidify within the cavity to form a casting and interrupting thefeed of metal to the cavity and removing the casting from the cavity,wherein the mould cavity comprises a moulding feature which has upperand lower surfaces for contact with the metal and which projectsinwardly of the mould cavity from a main wall thereof and has aconfiguration such that a mould made of 100% silica sand, except forusual impurities, and LM25 aluminum alloy is the metal cast, a part ofthe casting resulting from the moulding feature is displaced by at least5% from its designed position relative to the nearest part of thecasting defined by the main wall of the cavity made of sand whichcomprises at least substantially wholly sand selected from the groupconsisting of zircon sand and other particulate material suitable formaking a mould having a bulk density lying in the range 2-3 gm/cc.
 6. Amethod according to claim 5 wherein the moulding feature is formed ofunbonded sand.
 7. A method according to claim 6 wherein the unbondedsand comprises at least substantially wholly zircon sand, except forusual impurities.
 8. A method according to claim 5 wherein at leastsubstantially the whole of the moulding feature is preformed of sandbonded with a bonding agent.
 9. A method according to claim 8 whereinthe bonded sand comprises at least substantially wholly zircon sand andbonding agent except for usual impurities.
 10. A method according toclaim 5 wherein the pattern is supported within a container, by means ofa feed member which is mounted within the container, introducing thesand into the container to embed the pattern therein, and feeding metalinto the mould cavity through a passage provided by the feed member intothe mould cavity, an ingate part of the pattern being disposed incasting relationship with an orifice in the container and the metalbeing fed generally upwardly against the force of gravity to the orificefrom a reservoir of molten metal which is at a level which is below thelevel of the cavity.
 11. A method according to claim 5 wherein themoulding feature has upper and lower surfaces for contact with the metaland which projects inwardly of the mould cavity from a main wall thereofand has a configuration such that the length of any one section of themoulding feature is at least twice the thickness of the thinnest partthereof by which the section is connected to the main wall of the mouldcavity.
 12. A mould for making metal castings comprising consolidatedunbonded sand in which is embedded an in situ destroyable pattern todefine a mould cavity, there being provided in the mould cavity amoulding feature which has upper and lower surfaces for contact with themetal and which projects inwardly of the mould cavity from a main wallthereof and has a configuration such that a mould made of 100% silicasand, except for usual impurities, and LM25 aluminum alloy is the metalcast, a part of the casting resulting from the moulding feature isdisplaced by at least 5% from its designed position relative to thenearest part of the casting defined by the main wall of the cavity andcomprising at least substantially wholly sand selected from the groupconsisting of zircon sand and other particulate material suitable formaking a mould having a bulk density lying in the range 2-3 gm/cc.
 13. Amould according to claim 12 wherein the moulding feature is formed ofunbonded sand.
 14. A mould according to claim 13 wherein the unbondedsand comprises at least substantially wholly zircon sand, except forusual impurities.
 15. A mould according to claim 12 wherein at leastsubstantially the whole of the moulding feature is preformed of sandbonded with a bonding agent.
 16. A mould according to claim 15 whereinthe bonded sand comprises at least substantially wholly zircon sand andbonding agent except for usual impurities.
 17. A method of making metalcastings comprising the steps of providing an in situ destroyablepattern, then embedding the pattern in unbonded sand and consolidatingthe sand to form a mould in which is defined a mould cavity, thenfeeding a molten metal selected from the group consisting of aluminumand aluminum alloys into the cavity and permitting the metal to solidifywithin the cavity to form a casting and interrupting the feed of metalto the cavity and removing the casting from the cavity, with at leastpart of the mould comprising at least one preformed moulding featurewhich is included in said pattern and being formed of sand whichcomprises at least substantially wholly sand selected from the groupconsisting of zircon sand and other particulate material suitable formaking a mould having a bulk density lying in the range of 2-3 gm/cc.